In the manufacture of semiconductor devices and other articles through coating or etching processes performed on wafers and other substrates, it is imperative that the contamination, by microscopic particles, of the substrate surfaces to be processed be minimized. Surfaces to be protected from such contamination include, for example, the device surfaces of semiconductor wafers. The device surfaces are the surfaces of the wafers on which layers of conductive, insulative or other material are coated or etched by sputtering or other processes to form the manufactured devices.
In such processes, the presence of minute microscopic particulates on the device surface of a wafer may render an entire device functionally defective by adversely affecting the application or removal of a component layer at a critical point on the wafer surface. Similarly, magnetic disks, optical disks, lenses, magneto-optical disks and other such objects may be substantially reduced in value or quality where the number of particulates that contact the surface during processing is high. In the manufacture of large scale integrated circuits, a large quantity of semiconductor devices is formed of a single wafer. In the processing of such wafers, the number of microscopic particles present on the device surface of a wafer during processing significantly reduce the number of such devices of acceptable quality produced.
In a semiconductor processing apparatus such as a typical sputter coating or sputter etching machine, a wafer substrate is processed in an isolated and usually near vacuum environment. Such machines have a vacuum chamber in which the processing operations are carried out. The vacuum chamber is provided with one or more chamber doors located in the wall of the chamber through which wafers being processed are introduced and removed. On the outside of this chamber, the wafers are moved by some wafer handling mechanism between a cassette or carrier and the chamber door. A transfer mechanism which is usually included in the external wafer handling mechanism introduces the wafers into and removes them from the vacuum chamber through the chamber door opening. In the vacuum chamber, the wafer is usually received and held by a holder that supports the wafer during processing.
During the entry and removal of the wafers from the vacuum chamber, the portion of the chamber into which the wafers are placed and from which the wafer is removed will necessarily be at the same pressure and of the same atmospheric environment as exists in the external environment outside the chamber door. During processing, however, the portion of the chamber in which the wafer is to be processed must be brought to the vacuum pressure and atmospheric content as the process requires. This change of atmospheres necessitates a repetitive opening and isolation of the internal and external environments and the alternate pumping and venting of at least a portion of the vacuum chamber.
Wafer processing machines that process wafers in a vacuum environment sometimes maintain a constant vacuum environment so that processing upon some wafers can be carried out as others are being inserted into or removed from the chamber. To this end, such machines have an intermediate chamber or load lock at the entry to the processing chamber that alternately communicates with the external environment through the open chamber door, and, when the door is sealed, with the internal environment of the processing chamber through a sealable entry thereto. Such a load lock is alternately pumped to the vacuum level of the internal environment of the main processing chamber and vented to the external environment so that the pressure of the load lock matches that of environment with which it communicates during the introduction into and removal from the processing chamber of the wafers. With such a load lock, the internal portion of the chamber where processes are carried out, may be maintained continuously at the pressure and composition of the vacuum environment and, may be used additionally, for the processing of other wafers while wafers are being introduced into and removed from the load lock chamber.
Some processing machines are provided with two load lock chambers, one for the introduction of unprocessed wafers into the main chamber and one for removal of processed wafers from the main chamber. To avoid delay of processing operations being carried out in a processing chamber while wafers are being exchanged through the load locks, each of the operations of pumping the chamber and venting the chamber must take place in one machine or processing cycle. Where a single load lock is provided, to avoid such delays both entry of wafers into and removal of wafers from the apparatus must occur sequentially in one such machine cycle. This cycle must encompass the venting of the chamber to the pressure of the external environment, the opening of the load lock chamber door, exchange of a processed wafer for an unprocessed wafer in the load lock, the closing of the chamber door, and a pumping of the load lock to the vacuum pressure level of the internal environment of the processing chamber.
Often, the wafer processing involves a number of processing steps, each performed at a different processing station within a main chamber of a single wafer processing apparatus. In such an apparatus, different processes may be performed simultaneously at different processing stations differently on different wafers. The duration of these steps may be viewed as one machine cycle. At the beginning of each cycle, the load lock doors to the external environment are closed, the load lock chamber is pumped to the vacuum level of the processing chamber and the load lock or load locks are opened to the main chamber. An unprocessed wafer is supported in a holder in the load lock chamber while one or more wafers, once fully processed, is held in the main chamber at a final processing station. With the main chamber open to the load lock or load locks, one unprocessed wafer is moved into the main chamber from a load lock while one fully processed wafer is moved from the main chamber to the same or another load lock. At the end of each cycle, the load lock chamber is isolated from the main chamber, vented to the external environment and then opened to the external environment so that the processed wafer can be removed.
The operation of the load lock of a wafer processing machine having a single load lock proceeds in a cycle that begins with an opening of the load lock to the main chamber, the movement of a processed wafer from the main chamber to the load lock chamber where it replaces an unprocessed wafer which is simultaneously moved to the main chamber from the load lock chamber, and the sealing of the load lock to isolate the load lock chamber the internal processing environment of the main chamber so that processing in the main chamber can proceed. Once sealed, the load lock chamber is vented to the external atmosphere at a flow rate which allows for the opening of the load lock door, exchange of a processed wafer for an unprocessed wafer and the pumping of the load lock back to the vacuum pressure level during the machine process cycle. Both the pumping and the venting of the load lock cause a turbulent flow of gas within the load lock chamber.
In this and other types of processing machines, gases are pumped and vented into or out of processing and other chambers in which wafers or other substrates are being held before, during or after processing. Turbulent gas flow is often unavoidable in such chambers. In chambers such as processing and etching chambers, for example, a turbulent flow of gas during pumping or venting may occur. In these chambers, too, particulates are disturbed and may move to surfaces of objects which must be protected from such particulate contamination.
In the course of gas flow into and out of the load lock and other chambers, minute microscopic particulates, which have unavoidably collected on surfaces within the chamber, are disturbed by the turbulence of the flowing gas. The turbulently flowing gas picks up and carries the disturbed particulates about the chamber. In the prior art, many of these particulates come into contact with the device surface of the wafer within the chamber. This turbulent redistribution of particulates about the load lock chamber occurs when the vented gas is introduced into the chamber prior to the removal of a processed wafer from the chamber, and in addition, when gas is pumped from the chamber after an unprocessed wafer has been inserted into the chamber prior to the processing of the wafer.
The contact of particulates with the device surfaces of wafers results in adhesion of the particles which in turn causes the likely formation of defective devices as the particles interfere with the coating and etching processes and the deposition and removal of semiconductor layers on the wafer.
Heretofore in the prior art, the effort to minimize the particle contamination of the surfaces of wafers and other such substrates has focused on the meticulous and thorough cleaning of the chambers and on efforts to maintain an ultraclean environment surrounding the apparatus to thereby reduce the introduction of contaminating particulates into the chambers. Such efforts have resulted in expensive and time consuming solutions to the particulate contamination problem.
Accordingly, there is a need to reduce the particulate contamination of semiconductor wafer disks, and other objects in load locks and other pressure chambers of processing machines when turbulent gas is flowing therein and to render the cleaning of chambers and the maintenance of a clean environment in the vicinity of machines employing such chambers less expensive and less critical.